Hydrocarbon production from some reservoirs may require some level of assistance where the reservoir pressure is insufficient to support natural lift, for example due to depleting reservoir pressure following a period of production, and/or where an operator requires higher flow rates than can be naturally supported. In some cases a fluid may be injected which modifies one or more properties of the reservoir fluid to improve its ability to flow to surface. This might involve injecting a fluid which has the effect of reducing the density of the fluid and thus the weight of the fluid column within the wellbore, enabling or assisting the available pressure to lift the fluid column to surface. This may be achieved by injection of a gas, diluent, foaming agent or the like.
In so called gas-lift applications a gas, such as a hydrocarbon gas, is delivered from surface at high pressure and injected into a production string at one or more locations. In many applications gas is delivered from surface via an annulus surrounding the production string, and enters the production string via specialised completion equipment, such as gas lift mandrels and gas lift valves mounted in the mandrels. The gas lift valve typically functions to permit inward flow of the gas, while preventing or checking any reverse flow.
In some known gas lift valves it may be possible to control a flow orifice within the valve. Examples of such variable orifice valves are disclosed in, for example, U.S. Pat. Nos. 5,896,924, 5,937,945, 6,070,608, 6,148,843, 5,971,004, and 6,082,455.